Pumps having a variable displacement capability are commonly used in many different types of hydraulic systems. Some machines, such as, for example, vehicles, commonly include hydraulic pumps that are driven by an engine or motor in the machine to generate a flow of pressurized fluid, such as, for example, oil. The pressurized fluid may be used for any of a number of purposes during the operation of the vehicle. A machine, for example, may use the pressurized fluid to propel the machine around a work site or to move an implement on the machine.
A variable displacement pump is well known in the industry to drive a hydrostatic motor. In a machine having a variable displacement pump and a hydrostatic motor, such as an excavator or a loader, the pump provides fluid to the hydrostatic motor and drives the motor in the forward or reverse direction. The speed of the hydrostatic motor is controlled by varying the displacement of the pump.
A variable displacement pump generally includes a drive shaft, a rotatable cylinder barrel having multiple piston bores, and pistons held against a tiltable swashplate. When the swashplate is tilted relative to a longitudinal axis of the drive shaft, the pistons reciprocate within the piston bores to produce a pumping action. Each piston bore is subject to intake and discharge pressures during each revolution of the cylinder barrel.
The pump may include a mechanism that varies the angle of the swashplate to change the stroke length of the pistons and thereby vary the displacement of the pump. The displacement of the pump may be decreased by changing the angle of the swashplate to shorten the stroke length of the pistons. Alternatively, the displacement of the pump may be increased by changing the angle of the swashplate to increase the stroke length of the pistons.
Similarly, a hydrostatic motor includes an output drive shaft, a rotatable cylinder barrel having multiple piston bores, pistons disposed in the bores, and a swashplate. The pistons reciprocate within the piston bores to produce a pumping action. Each piston bore is subject to intake and discharge pressures during each revolution of the cylinder barrel. The pumping action by the pistons rotates the cylinder barrel and the output drive shaft, thereby providing a motor output torque.
The amount of pressurized fluid required from a variable displacement pump may vary depending upon the particular operating conditions of the system or vehicle that relies upon the pump. In a vehicle application, the overall efficiency of the vehicle may be improved by varying the displacement of the pump to match the requirements of the vehicle. For example, if the vehicle requires less pressurized fluid, the angle of the swashplate may be changed to decrease the stroke length of the pistons. If the vehicle requires more pressurized fluid, the angle of the swashplate may be changed to increase the stroke length of the piston.
A vehicle or system may include a control system that monitors one or more operating requirements and controls the operation of the pump to match the requirements. To effectively match the output of the pump with the requirements of the vehicle or system, the control system monitors an output of the pump.
One method of monitoring the output of the pump is described in U.S. Pat. No. 4,510,750 (the '750 patent) to Izumi et al. The '750 patent describes a circuit pressure control system for hydrostatic power transmission. The control system adjusts a tilt angle of a swashplate of a hydraulic pump, thereby adjusting a pressure in a circuit interconnecting the hydraulic pump and a hydraulic motor. The control system determines a swashplate tilt command based on a measured circuit pressure and a preset pressure value. The swashplate tilt command is then compared to a measured swashplate tilt angle. The difference between the measured and commanded swashplate tilt angle is used to determine an operating current that is supplied to a servo valve. The servo valve controls a displacement adjusting mechanism that adjusts the tilt angle of the swashplate, thereby adjusting the circuit pressure.
Although the control system of the '750 patent may control the circuit pressure, it may be complex because it determines the operating current based on the measured swashplate tilt angle. Therefore, the control system of the '750 patent includes a swashplate tilt angle detector. Furthermore, the control system determines the operating current based on the difference between the measured and commanded swashplate tilt angles. Therefore, the control system determines the tilt angle of the swashplate in order to adjust the circuit pressure.
The disclosed system is directed to overcoming one or more of the problems set forth above.